TAPSOFT '91 is the Fourth International Joint Conference on Theory and Practice of Software Development. It was held in Brighton, April 8-12, 1991, and was organized by the Department of Computing, Imperial College, London. The proceedings of TAPSOFT '91 are organized into three parts: - Advances in Distributed Computing (ADC) - Colloquium on Trees in Algebra and Programming (CAAP) - Colloquium on Combining Paradigms for Software Development (CCPSD) The proceedings are published in two volumes. The first volume (LNCS, Vol. 493) contains the papers from CAAP. The second volume (LNCS, Vol. 494) contains the papers from the ADC and CCPSD. The ADC talks by distinguished invited speakers surveys current developments in distributed computing, including the integration of different paradigms for concurrency, algebraic, logical and operational foundations, and applications to software engineering and formal methods. The CCPSD papers address aspects of the trend in software enginering towards unification and synthesis combining theory and practice, and merging hitherto diverse approaches.